Here goes the backtrace, for the record and evidence, just in case the pastebin.com snippet expires: the crash is indeed in mdaEPiano native VST plug-in, no doubt about it, but being it at the main synthesis process method (processReplacing), it suggests that there's some kind of corruption incident in or near the plugi-in address space vicinity (which is Qtractor's nevertheless).
$ gdb /usr/bin/qtractor core
...
Core was generated by `qtractor dub-hop-noclaf.qtr'.
Program terminated with signal 11, Segmentation fault.
...
(gdb) bt
#0 0xb64509bc in mdaEPiano::processReplacing ()
from /samples/vst/linux-vst/mdaEPiano.so
#1 0xb644a29c in AudioEffect::processClassReplacing ()
from /samples/vst/linux-vst/mdaEPiano.so
#2 0x08172816 in ?? ()
...
(gdb)
Here goes the backtrace, for the record and evidence, just in case the pastebin.com snippet expires: the crash is indeed in mdaEPiano native VST plug-in, no doubt about it, but being it at the main synthesis process method (processReplacing), it suggests that there's some kind of corruption incident in or near the plugi-in address space vicinity (which is Qtractor's nevertheless).